A series of proline analogs have been analyzed for their effects on collagen synthesis inhibition in cultures of primary DMBA-induced rat mammary tumors and for their effects on mammary tumor growth in tumor bearing animals. Azetidine carboxylate, thioproline and cis-hydroxyproline were found to be potent, selective inhibitors of collagen synthesis, blocking amino acid incorporation into collagen by 7 to 27 fold more than incorporation into total tumor cell protein. In vito all 3 of these compounds at doses of 50-200 mg/kg S.C., caused tumor growth arrest or regression. The conditions favoring proline analog sensitivity of mammary tumors have been assessed. A positive correlation exists between the ability of a tumor to synthesize basement membrane and its analog sensitivity. The analogs do not produce any discernable, general toxicity at concentrations which affect tumor growth. Sensitivity is approximately proprotional to the efficacy of the analog in blocking collagen synthesis in culured tumor epithelium. The epithelium of normal mammary glands and mammary adenocarcinomas is dependent on proline for optimal growth, especially when cells are plated on stromal collagen substrata. Blocking basement membrane deposition and thereby favoring tumor cell contact with stroma may, therefore, promote proline analog uptake and tumor cell kill.